Silver conductors



Nov. 22, 1960 J. A. BALDREY ET AL 2,961,416

SILVER CONDUCTORS Filed June 9, 1958 INVENTORS JOHN A. BALDRE Y OLIVERA. SHORT ATTORNEY United States Patent SILVER CON DUCTORS John A.Baldrey, Perth Amboy, and Oliver A. Short, Me-

tuchen, N.J., assignors to E. I. du Pont de Nemours and Company,Wilmington, D'el., a corporation of Delaware Filed June 9, 1958, Ser.No. 740,959

2 Claims. (Cl. 252-'-514) This invention rel-ates to silver electricalconductors, and more particularly it relates to silver conductorsprovided with means to substantially diminish the tendency of the silverto migrate. 7

Silver is used in many forms in electrical equipment. Due to its lowarcing tendency and self-cleaning ability, it is frequently used forswitch contacts. Because it is h'ghly corrosion-resistant and itscorrosion products are either electrically conducting or easily removed,it is also used in the form of silver electroplated brass connectorssuch as the pin contactors in tube sockets.

Silver is highly conducting and can be deposited as a fine powder with aglassy binder on ceramic surfaces, making it useful for capac'tors andelectric printed circuits. It can also be printed with organic binderson phenolic or other resin surfaces to form electrically conductingcircuits. When two silver conductors are in close proximity to eachother and in an electric circut where one is cathodic and the otheranodic, metallic silver will migrate from the cathodic conductor to theanodic conductor in the presence of moisture. The silver will build upon the cathode to gradually cross the gap between the cathode and anodeand thereby eventually create a short circu't. This silver migration isdue to the relatively high solubility of silver hydroxide in a moisturefilm and the ease of electrodeposition of metallic silver. The migrationapparently takes place through the following mechanism. Where a film ofmoisture is present on the electric c'rcuit comprised of the twooppositely charged conductors positioned on a dielectric base, hydroxylions from the ionization of water migrate to the anodic elements,dissolving silver from the anode as silver hydroxide. The silver ionsproduced by ion'zation of silver hydroxide migrate through the layer ofmoisture to the cathode where they are deposited as metallic silver.Subsequent electrodeposits form at the outer edge of former silverdeposits and thereby form a chain of metallic silver eventuallyextending across the gap between the oppositely charged electricalconductors.

Many attempts have heretofore been made to overcome the migration ofsilver in silver electrical conductors, for example, by prov'ding goldor platinum barriers on the side of the anode closest to the cathode todischarge hydroxyl ions before the silver is attacked. In some instancesgold or platinum have been used as the electrical conductor to avoid thebad elfects of silver migration. Such use of gold or platinum is far tooexpensive to be used except in very exceptional cases.

It is an object of this invent'on to greatly diminish or substantiallyeliminate the objectionable migration of silver between silverelectrodes positioned on a dielectric base.

it is another object of this invention to produce greatly improvedelectrical circuit contacts comprised of silver conductors.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in

2,961,416 Patented Nov. 22, 1960 general, by placing into contact withthe anodic silver electrode a small quantity of metallic zinc ormagnesium. The zinc or magnesium mustbe electrically contacted with thesilver anode. When the applied voltage is alternating, making one of theelectrical elements cathodic atone instance and anodic at the next,protective metal pieces must contact each electrode. The zinc ormagnesiurn may be mechanically attached by means of a foil clamped ontothe silver electrode, these metals may be plated onto the silverelectrode, the magnes'um may be alloyed with the silver, or, in the caseof silver electrodes made from finely divided silver, the magnesium maybe incorporated in finely divided form with the finely divided silverand applied to the dielectric to form the silver electrode.

It appears that the magnesium or zinc metal will form magnesiumhydroxide or zinc hydroxide preferentially to the formation of silverhydroxide. The hydroxides of magnesium and z'nc, however, are veryinsoluble and do not tend to ionize and cause migration.

Peculiarly not all metals more electroactive than silver will properlyfunction as agents to prevent silver migration. Alkali metals are, forexample, far too active. Antimony, aluminum, chromium, iron, n ckel,beryllium, cadmium, iridium, manganese, tantalum, thallium, selenium,lead, bismuth, titanium, and vanadium have all been tried and failed tofunction to prevent the mgration of s'lver.

The details of the invention will be more clearly apparent by referenceto the following description when taken in connection with theaccompanying drawings showing certain illustrative embodiments of theinvention, and inwhich Figures 1 and 2 are diagrammatic perspectiveviews of oppositely charged silver electrodes on a dielectric base;

Figures 3 and 4 are similarly diagrammatic perspective views ofoppositely charged silver electrodes with means to prevent silvermigration.

Referring to the figures, 1 designates a dielectric base material whichmay consist of a ceramic such as glass, barium titanate, or the like, orit may consist of an organ'c plastic material such as phenolformaldehyde, urea formaldehyde, or the like. Two silver patternsnumbered 2 and 3 are positioned on the dielectric 1 and function asoppositely charged electrodes in an electric circuit. 4 discloses thetree-like deposit of silver on the cathode upon moisture contact betweenthe two electrodes.

In Figure 2 the silver deposits have progressed from the cathode untilthey have bridged the gap between the cathode and the anode at 5. Uponbridging the electrodes, the current will be short-circuited.

Figure 3 again discloses similarly oppositely charged silver electrodesand a small foil 6 of magnesium or zinc positioned in electrical contactwith the anode. It is not essential that the foil cover the tip or edgeof the silver electrode exactly as shown, but may be positioned backfrom the edge if convenient. Exposure of a large area of silver betweenthe foil and the cathode is, however, undesirable. As a result of thepresence of this magnesium or zinc, the hydroxyl ions of the ionizedwater will preferentially form magnesium or zinc hydroxide andsubstantially diminish or prevent the migration of silver.

In Figure 4 the particles of magnesium or Zinc 7 are shown incorporatedin the body of the silver electrodes and likewise prevent or at leastvery substantially diminish the migration of silver from the anode tothe cathode.

The following examples are given to illustrate the operation of theinvention in accordance with several of the most desirable embodiments.

Example I mediately and in less than 5 seconds bridge the i -inch V gapcausing an electric short circuit.

Example II Example 11 is repeated using a small piece of magnesium foilin place of the zinc foil. Upon applying the same electric potential, nomigration was observed in over minutes. This represents a 100-fold orgreater improvement.

Example IV Example ii is repeated using foils composed of antimony,aluminum, chromium, iron, nickel, beryllium, cad- 1 mium, iridium,manganese, tantalum, thallium, selenium, lead, bismuth, titanium andvanadium. In each case the migration was substantially as rapid as inExample I without the use of any foil.

Example V A silver paste was prepared similarly to that of Example I,but approximately 1 part of finely divided powdered magnesium metal wasadded for each 9 parts of finely divided silver. Themagnesium-containing silver was then printed and tested as in Example I.No migration was observed after more than 10 minutes exposure to theelectric potential of 12 volts D.C.

Example VI Silver plated brass contactors were molded into a phenolresin base. A water film was placed between two adjacent contactors anda l2-vo1t DC. potential applied across to adjacent contacts. The silveraccumulated on the cathode and bridged the gap between the twocontactors in an exceedingly short time. No improvement over the silvermigration of Example I was noted.

Example VII Silver plated brass contactors were molded into a phenolresin base similarly to Example VI. However, before molding thesecontactors into the phenol resin, they were plated with magnesium. Thismay be done from a pyridine bath or from a molten magnesium chloridebath. It is essential that the magnesium be in electrical contact withthe silver and that it be exposed at the phenol resin-air interface. Awrap of thin magnesium foil may be used instead of the electroplate. Adrop of water is placed between the two exposed portions of theelectrical contactors and a 12-volt DC. potential applied. No migrationtakes place over a period of 10 to 30 minutes.

Example VIII Finely divided silver in a vitreous enamel binder isprinted and fired on barium titanate wafers in accordance with theteaching of the method disclosed in Knox U.S. Patent 2,385,580. Two suchprinted silver conductors -lYlCh apart are charged with a 12-volt DO. Adrop of water placed on the barium titanate wafer between the two silverelectrodes evidences silver migration to the point where the gap betweenthe electrodes is bridged with a period of about 5 seconds.

Example IX Silver electrodes are printed and fired on a barium titanatewafer in accordance with the process of Example VIIl except that 10% byweight of magnesium powder is first added to the finely divided silver.The two electrodes, between which exists a 12-volt DC. potential, failto show any migration with water covering both electrodes. In firing thesilver containing the magnesium powder the temperature must be heldfairly low and the oxygen present reduced as much as possible to avoidoxidation and removal of the magnesium from the silver.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that this invention is not tobe limited to said details except as set forth in the appended claims.

We claim:

1. An electrical conductor composition comprising to by weight of finelydivided silver and 5% to 30% by weight of a finely divided metal fromthe group consisting of magnesium and zinc, said finely divided metalsbeing dispersed together in a vehicle.

2. An electrical conductor composition comprising 70% to 95% by weightof finely divided silver and 5% to 30% by weight of a finely dividedmetal from the group con sisting of magnesium and zinc, said finelydivided metals being dispersed in a thermosetting binder.

References Cited in the file of this patent UNITED STATES PATENTS

1. AN ELECTRICAL CONDUCTOR COMPOSITION COMPRISING 70% TO 95% BY WEIGHTOF FINELY DIVIDED SILVER AND 5% TO 30% BY WEIGHT OF A FINELY DIVIDEDMETAL FROM THE GROUP CONSISTING OF MAGNESIUM AND ZINC, SAID FINELYDIVIDED METALS BEING DISPERSED TOGETHER IN A VEHICLE.